API 20E Spec

API 20E bolting refers to American Petroleum Institute standards for high-performance fasteners used in critical oil & gas applications, defining three levels (BSL-1, BSL-2, BSL-3) with increasing quality, testing, and traceability requirements for preventing catastrophic failures in harsh environments. These standards ensure alloy/carbon steel bolts resist corrosion, hydrogen embrittlement, and stress cracking, covering material sourcing to final product for subsea and surface equipment. 

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Key Aspects of API 20E Bolting

• Purpose: To provide enhanced quality and control for bolts in high-pressure, harsh environments (offshore, subsea) where failure risks are significant, preventing leaks and ensuring safety.

• Bolting Specification Levels (BSLs):

  • BSL-1: Baseline for low-risk applications, requiring standard material specs and QC.

  • BSL-2: Medium-risk; more rigorous testing (NDT, traceability).

  • BSL-3: Highest criticality (e.g., preventing hydrocarbon release); mandates extensive testing (volumetric NDT on every bolt) and strict manufacturing control.

• Manufacturing Focus: The standard controls the manufacturing process, not just post-production testing; compliance must be built in from raw material to finished product.

• Material & Types: Covers alloy and carbon steels (e.g., ASTM A193, A320, A194 grades), used for stud bolts, nuts, etc., in equipment like choke & kill manifolds.

• API Monogram: Certified API 20E fasteners bear the API monogram, signifying compliance. 

Bolting Grade

Grade B7: The High-Strength Workhorse

ASTM A193 Grade B7 is the most prevalent specification for high-pressure, high-temperature bolting in the energy industry.  

• Material: The raw material for Grade B7 studs and bolts is AISI 4140, a chromium-molybdenum (Cr-Mo) alloy steel.8 This alloy is chosen for its excellent combination of high tensile strength, toughness, and resistance to creep at elevated temperatures.

• Heat Treatment: To achieve its desired mechanical properties, the AISI 4140 steel undergoes a quench and temper process.9 The material is heated to an austenitizing temperature (typically around 845∘C), rapidly quenched in a liquid medium (like oil or water), and then tempered at a minimum temperature of 593∘C (1100∘F). This process refines the grain structure, imparting high strength while retaining necessary ductility.

• Mechanical Properties: Grade B7 bolts are characterized by a minimum tensile strength of 125 ksi (860 MPa) and a minimum yield strength of 105 ksi (720 MPa) for diameters up to 2.5 inches.

• Applications: B7 bolts are the standard choice for connecting flanges, valves, and pressure vessels in refineries, petrochemical plants, and upstream production facilities operating under non-sour conditions.

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Grade B7M: Engineered for Sour Service

ASTM A193 Grade B7M is a modification of Grade B7, specifically designed for resistance to sulfide stress cracking (SSC) in environments containing wet hydrogen sulfide (H2​S), commonly known as “sour service.”

• Material: The material is also AISI 4140 alloy steel.10

• Heat Treatment & Testing: While the quench and temper process is similar, the tempering temperature is higher, and the cooling rate is strictly controlled. The primary distinction of B7M is the 100% hardness testing requirement after heat treatment. Every single B7M fastener must be tested to ensure its hardness does not exceed 235 HBW (Brinell Hardness) or 99 HRB (Rockwell Hardness). This lower hardness limit is critical because high-strength steels with high residual stresses are highly susceptible to SSC.

• Mechanical Properties: The controlled heat treatment results in lower strength properties compared to B7. The maximum tensile strength is capped at 120 ksi (827 MPa) and the minimum yield strength is 80 ksi (550 MPa). This trade-off between strength and SSC resistance is fundamental to ensuring material integrity in sour environments.

• Applications: B7M is mandated for critical pressure-containing equipment exposed to sour gas in wellheads, pipelines, and processing facilities.

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Grade L7: Toughness at Low Temperatures

ASTM A320 Grade L7 bolting is designed for low-temperature service where brittle fracture is a primary concern.

• Material: Like B7, Grade L7 is typically manufactured from AISI 4140 alloy steel.11

• Heat Treatment & Testing: The key differentiating factor for Grade L7 is the mandatory Charpy V-notch impact test. This test measures the material’s ability to absorb energy at a specified low temperature, ensuring its toughness and resistance to brittle failure. For Grade L7, the impact test is conducted at −101∘C (−150∘F). The material must meet a minimum average absorbed energy value to qualify.

• Mechanical Properties: The strength properties of L7 are identical to those of Grade B7 (125 ksi tensile, 105 ksi yield). The manufacturing process focuses on achieving this strength while guaranteeing low-temperature toughness.

• Applications: L7 fasteners are essential for cryogenic applications, such as LNG (liquefied natural gas) facilities, and for oil and gas equipment deployed in arctic or deep-water environments where ambient temperatures can fall well below freezing.

Grade L7M: The Ultimate for Low-Temp Sour Service

ASTM A320 Grade L7M combines the requirements of both low-temperature toughness and sour service resistance. It is one of the most highly engineered grades.

• Material & Manufacturing: Made from AISI 4140, L7M undergoes a specialized quench and temper cycle.12 It must meet the hardness restrictions of Grade B7M (maximum 235 HBW) to prevent SSC, and simultaneously pass the Charpy V-notch impact test requirements of Grade L7 at −101∘C (−150∘F) to prevent brittle fracture.

• Mechanical Properties: The mechanical strength properties are the same as for B7M (80 ksi minimum yield strength).

• Applications: L7M is specified for the most demanding service conditions: equipment that is exposed to both sour gas and extremely low temperatures. This includes critical wellhead and BOP components in harsh environments like the North Sea or Alaska’s North Slope.

The most critical step is the heat treatment, performed in precisely calibrated furnaces to achieve the specific microstructure and mechanical properties for each grade.

Following heat treatment, the fasteners are threaded, typically through a roll threading process that enhances fatigue resistance compared to cut threading. Finally, they may be coated with protective finishes like Zinc, Cadmium, or specialty fluoropolymers (e.g., PTFE) to enhance corrosion resistance.

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Quality Control is embedded in every step. A battery of tests is performed in their in-house, NABL-accredited laboratory:

• Tensile Testing: To verify yield strength, tensile strength, and elongation.

• Hardness Testing: Performed on all B7M/L7M bolts and on a sample basis for B7/L7.

• Impact Testing: Charpy V-notch tests for all L7 and L7M heats.

• Non-Destructive Testing (NDT): Magnetic Particle Inspection (MPI) to detect surface-breaking flaws and Ultrasonic Testing (UT) to find internal defects.

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Every shipment is accompanied by a comprehensive Material Test Report (MTR) and a certificate of conformance, providing the end-user with complete documentation and confidence in the product’s integrity.